DBA2 skin grafts placed on BALB/c mice are uniformly rejected although the two strains are identical H-2d. Despite this we have found two situations in which organ grafts from DBA2 to BALB were accepted. First, thyroid grafts transferred from DNA2 to BALB underwent a period infiltration, which peaked at three weeks and then cleared up by 6 weeks. In 75% of the transplants a functioning thyroid graft was retained as indicated by 125I uptake and a characteristic histology. At this time the transplantation of B6D2 thyroids or skin grafts was followed by rejection of the latter grafts at the expected time (10-12 days). Despite the rejection of the B2D2 grafts, the DBA2 thyroid grafts applied earlier were obtained. Another situation where unexpected tolerance of DBA2 grafts was obtained was found in radiation chimeras after destruction of the chimera. BALB/c mice were first made chimeras by injecting DNA2 bone marrow after 750 gamma,gamma-radiation. DBA2 skin grafts were applied and accepted. At this point spleen cells from BALB/c mice which had rejected DNA2 skin grafts were injected. Although the chimera was destroyed by this injection, the DBA2 skin graft was retained. Furthermore, the rejection of a new B2D2 skin graft did not result in a rejection of the DBA2 graft. In both situations described above the rejection of a hybrid graft containing both major and minor antigenic differences with the host did not result in rejection of a graft containing only minor antigens. We believe that the reason for this failure may be that many minor antigens are not expressed on every cell. Stated another way the minor antigens found on stimulating cells may not be found on non-stimulating cells.